JP-A-8-37789 (Patent Document 1) discloses a control method for an AC motor in which a switching frequency is varied in accordance with the value of a smoothened DC power supply voltage, the rotational speed of the AC motor, a primary electrical angle frequency, the intensity of an output voltage to be supplied or the frequency value of the output voltage or output current. JP-A-10-229674 discloses a DC-DC converter for carrying out the control of reducing a switching frequency under an over-current condition. JP-A-2001-161065 discloses a switching power supply device in which the oscillation frequency of a PWM control means is continuously reduced as the load condition is reduced so that the power loss is minimized.
For example, in a fan motor driving device for supplying a desired voltage to a fan motor mounted in a vehicle by driving a semiconductor switching element in the PWM mode, the semiconductor switching element is overheated when the temperature of the environment surrounding the driving device is abnormally increased or the load torque of the fan motor is abnormally increased. In order to protect the semiconductor switching element from such an overheat state, there has been hitherto used a method of turning off the switching element on the basis of a signal from a temperature detecting sensor or current detecting sensor or reducing the duty ratio of PWM.
However, when the switching element is turned off, the fan motor is completely stopped and thus the blowing of cooling air is also stopped. Furthermore, even when the duty ratio of PWM is reduced, the rotational speed of the fan motor is reduced, and thus sufficient blowing of cooling air is unachieved. As a result, there occurs such a secondary trouble that a cooling target object is overheated or passengers are made to feel uncomfortable. That is, the fan motor is strongly required to be continuously operated with instructed output power.